Electrical Capacitance Tomography (ECT) is the reconstruction of material concentrations of dielectric physical properties in the imaging domain by inversion of capacitance data acquired by a capacitance sensor. Electrical Capacitance Volume Tomography (ECVT) is the direct three-dimensional (3D) reconstruction of volume concentration or physical properties in the imaging domain utilizing 3D features in the ECVT sensor design. Adaptive ECVT is an advanced technology that introduces a new dimension into 3D sensor design by applying voltages of various frequencies, amplitudes, and phases to different capacitance plate segments. Adaptive sensors can provide a much larger number of independent capacitance measurements of the imaging volume thereby enabling better resolution for the images.
Driven and grounded guards were developed to reduce or eliminate the fringe effect present in ECT sensors. Prior guards developed thus far are not flexible enough to cope with modern applications of capacitance tomography sensors.
The present invention relates to the novel use of active control guards with the objective of controlling the charge distribution on different capacitance electrodes and hence controlling the sensor sensitivity in a predictable manner. In the preferred embodiment, active control guards are segments that are electrically isolated from the sender or receiver plates from which capacitance is being measured. The active control guards influence the charge distribution on the sender and/or receiver plates without changing the capacitance between them. A change on the charge distribution on the sender and/or receiver plates allows for controlling the sensitivity map between any such pair of plates. This control is effected through a unique design and set of features explained below.
The present invention also relates to a new method of calculating the sensitivity matrix directly from the charge distributions on the sender and receiver plates. It also relates to estimating aggregate effect of multiple active control guards, when activated together, on sender and receiver charge distribution. This aggregate effect is estimated from a weighted summation of the effect of each control guard, when activated alone, on sender and receiver plates.
The present invention also relates to rationometric reconstruction and calibration that eliminates the conventional requirement of calibration in ECT and ECVT sensors based on the use of full and empty imaging domains. In the preferred embodiment, ECVT and AECVT plates are designed to have similar or rationometric capacitance values for a homogeneous medium.
The present invention also relates to using a reference capacitor in the measuring circuit to decode the effective dielectric permittivity between plates of the rationometric sensor.
The present invention also relates to using a recursive method for reconfiguring the active control guards patterns for each pair of capacitance plates used in data acquisition. In the preferred embodiment, recursive activation uses measured capacitance to reconfigure the impressed voltages at the active control guards with a new pattern so as to counter polarization in the dielectric material to maintain as close to a linear relation as possible between sensitivity distribution and dielectric distribution when dielectric distribution in the imaging domain evolves over time. Maintaining a nearly linear relation between charge distributions on measuring plates and dielectric distribution in the imaging domain, at all times, ensures sensor sensitivity is only affected at regions where dielectric distribution has changed. This allows for higher resolution imaging as the nonlinearity of the inverse problem is mitigated and changes in the capacitance signal can be directly traced back to locations in the imaging domain that caused the recorded changes in capacitance signals.